1. Field of the Invention
The invention relates to a system and process for removing paint from polymeric surfaces. The polymeric material is recycled.
2. Discussion of the Relevant Art
There are many processing operations where polymeric materials are coated with a film, sticker or other surface such as paint for industrial applications. As with any large scale processing operation, there inevitably will be a percent of `rejects` based on inferior surface or paint quality or improper color. Due to the integral relationship of the paint and the plastic, these products are normally scraped and disposed of in landfills or occasionally recycled in a low-end application where the new part is not painted or coated. If the material is recycled (ground and extruded through a screen and then remolded) small paint particles remain in the plastic and migrate to the surface during processing. These particles then create craters or bulges in the paint, thus developing another scrap part. Used polymers have hitherto been burned in an incinerated or buried together with other wastes under the ground. Combustion of such polymers increases carbon dioxide concentration in the air, leading to the serious problem of global warming. When they are buried under the ground, they cannot be decomposed by microorganisms and hence remain as they are, so that the amount of the polymers accumulated under the ground increases year by year.
In recent years, recycling of polymers has been advocated from the viewpoint of resource conservation and the earth's environmental protection.
However, various difficult problems should be solved in order to successfully recycle used polymers.
One of such problems is related to the fact that polymers as final products often are a composite of dissimilar materials, rather than a product of a single material. When such polymers as a composite are recycled without separation of the dissimilar materials, the recycled products often have inferior mechanical properties and appearance to those of the original polymers.
For example, when a coated polymer product, as such, is pulverized, repelletized with a pellet mill, pelletizer or heated extruder and remolded by means of a molding machine into a recycled polymer product, the coating which has been applied on the surface of the original polymer product remains in the recycled polymer product without being finely dispersed therein. Since the coating that cannot be finely dispersed inhibits homogenous blending of resins, the properties of the recycled product is considerably lowered.
Thus, it is of most importance to remove a coating from a coated polymer product in advance of its reprocessing for recycling.
One major area for the recycling of such a product is in the automotive industry for painted bumpers. These bumpers are typically a polymeric material having a paint applied thereto such that the bumper matches the color of the automobile.
Historically, recycling attempts have been made to simply melt the material down in a heated extruder, and capture the remaining paint particle contaminates in a screen or screen pack. This can be accomplished through a single screen pack or lining up multiple screens joined by a heated pump. Once the material has been through this type system, it might even be mixed with a similar clean material (copolymer in the auto bumper application) to reduce the amount of any remaining paint particles that have not been captured by the screening any dilution with the copolymer. This type system has proven to be slow and costly due to the high cost of good material that is used to dilute the subsequent recycled material.
Other approaches include chemical removal where parts or ground particles are treated with caustic chemicals that corrode the paint surface. This type process also has an adverse effect on the remaining plastic material that makes it difficult for reuse.
U.S. Pat. Nos. 5,788,811; 5,876,557 and 5,897,820 are all directed to the recycling of rejected polymeric materials having paint on the surface thereof each concentrating mainly on bumper parts. The objective of each of these patents is to separate the paint from the polymeric material whereby the polymeric material may be recycled.
In each of these patents, the primary mechanism by which the paint is removed is to shear or strip the paint from the underlying polymeric material by passing cut sections of the painted polymeric parts through a series of forming and shearing rolls, the shearing rolls having different peripheral speeds. This results in polymer strips that are then ground into smaller particles and subsequently melted down to make pellets for shipping or placed directly into a molding machine to form new parts.
Although not described in these references, a problem with this process is the efficient separation of the paint from the polymeric strips. That is, after the paint shearing step, basically there is an intimate mixture of residual paint particles that may adhere to the polymeric strips and subsequent particles. These paint particles adhere to the plastic by electrostatic bonds that are generated in any shearing or grinding process with plastics.
Also, these processes are labor intensive requiring parts be cut into sections and then fed into a series of rollers. Adding further to costs, parts must be handled initially in their "whole" state, requiring large volumetric logistics (large areas for shipping and/or material handling).